1. Field of the Invention
The invention relates to electrical energy storage. In particular, energy storage by reversible electrochemical oxidation and reduction of a molten metal is disclosed.
2. Background Information
Balancing supply and demand of electrical energy over time and location is a longstanding problem in an array of applications from commercial generator to consumer. The supply-demand mismatch causes systemic strain that reduces the dependability of the supply, inconveniencing consumers and causing loss of revenue. Since most electrical energy generation in the United States relies on the combustion of fossil fuels, suboptimal management of electrical energy also contributes to excessive emissions of pollutants and greenhouse gases. Renewable energy sources like wind and solar power may also be out of sync with demand since they are active only intermittently. This mismatch limits the scale of their deployment. Large-scale energy storage may be used to support commercial electrical energy management by mitigating supply-demand mismatch for both conventional and renewable power sources.
One approach to energy storage is based on electrochemistry. Conventional lead-acid batteries, the cheapest commercial battery technology on the market, have long been used for large-scale electrochemical energy storage. Facilities housing vast arrays of lead-acid cells have been used to provide high-capacity electricity storage, on the order of 10 MW. However these facilities are neither compact nor flexibly located. The short cycle life of lead-acid batteries, on the order of several hundred charge-discharge cycles, limits their performance in uses involving frequent activation over a wide voltage range, such as daily power management. The batteries do not respond well to fast or deep charging or discharging, which lowers their efficiency and reduces their lifespan.
Sodium-sulfur (“NAS”) batteries have been adapted to large-scale power management facilities in the US and Japan. An NAS battery incorporates molten sodium and sulfur electrodes opposed across a solid ceramic electrolyte. The electrolyte must be very thin in order to maximize sodium ion conduction, but this makes it mechanically fragile and imposes severe limits on the maximum size of an individual cell. This, in turn, affects scalability, i.e., large capacity must be achieved through many small cells rather than through few large cells, which greatly increases complexity and ultimately increases the cost of the system. Cell construction is complication by sodium's violent reaction with water and rapid oxidation in air.
There is, accordingly, a need for an energy storage device combining capacity, economy, flexibility and long life.